The present invention relates to an image forming apparatus of an electrophotographic method and more particularly to improvement of a developing art.
In recent years, there is an increasing need for high-speed performance and high-quality performance of an image forming apparatus of an electrophotographic method.
As an art for responding to such a need, a developing art using toner with a small particle diameter has been developed. Further, the particle diameter of a carrier is apt to become smaller in correspondence with use of toner with a small particle diameter.
The small particle diameter of the carrier not only corresponds to realization of the small particle diameter of toner but also suppresses irregularities of a half-tone image due to the small particle diameter of the carrier and can form an image with a smooth outline.
As mentioned above, the small particle diameter of the carrier is an advantageous means for forming a high-quality image, though in correspondence with realization of the small particle diameter, the magnetization of each particle of the carrier is reduced, thus a problem arises that the carrier is easily adhered. As a measure for this problem, that is, to prevent carrier adhesion, it is necessary to make the magnetization of the carrier larger. However, when the magnetization of the carrier is made larger, the bristles of the magnetic brush for executing development are raised, and the bulk density of the bristles is lowered, and as a result, a problem arises that a half-tone image is made uneven.
In Patent Document 1, to prevent an occurrence of irregularities, it is proposed to reduce the product of the particle diameter of the carrier and the magnetization to a fixed value or smaller and increase the magnetic flux density peak to a fixed value or larger.
[Patent Document 1]                Japanese Patent No. 3308681        
In the developing method of Patent Document 1, in the developing area, the image bearing member and developer bearing member are moved in the same direction, thus the development is executed. However, in the developing method for moving the image bearing member and developer bearing member in the same direction like this and executing the development, a problem that high density is hardly obtained and a problem that the carrier is easily adhered arise.
By a reversal developing method for moving the image bearing member and developer bearing member in opposite directions in the developing area and executing the development, such problems can be solved.
However, in the reversal developing method, it is found that irregularities are easily caused.